Current printing system typically use 4 colorants to compose color images, i.e. cyan, magenta, yellow and black. Among them, cyan, magenta and yellow can be denoted as primary colors because they can theoretically cover the entire printer color gamut. Black is further introduced to improve the stability of neutral rendition. The size of the achievable color gamut is determined by the chromaticity/saturation of the primary colors. As a result, a set of primary colors with higher saturation is able to produce more colorful images, which in turn are more preferred by viewers. However, all printing processes have their intrinsic noise, and it will manifest into various macroscopic and microscopic artifacts, such as granularity and mottle.
Researchers have found that, under the same printing noise environment, the perceived graininess is proportional to the luminance contrast of selected colorants (see Chung-Hui Kuo, Yee Ng, and Di Lai, Grain Profile of a Printing System, IS&T NIP23, September 2007). As a result, the manufacturers of printing presses have to strike a balance between the size of the color gamut and severity of granularity.
Generally there exist two approaches to address this issue: improve the printing process noise, and/or augment the current printing process with extra light color(s) with lower pigment concentration (See Chingwei Chang, U.S. Pat. No. 6,765,693, July 2004; and Yasukazu Ayaki, Takeshi Ikeda, Yukio Nagase, Nobuyuki Itoh, Isami Itoh, and Tomohito Ishida, U.S. Pat. No. 6,996,358, February 2006). An advantage of introducing supplemental light color(s) into a printing process is that it improves the color resolution capability so as to reduce possible color contouring problems. However, granularity is still a problem especially when there is a lower percentage coverage of color separation with current 8 micrometer toner. Even with smaller particle toners (such as 6 micrometer toners), variation in transfer efficiency with low coverage causes higher grain, especially in photo-rich applications that may involve enhanced gloss.
U.S. Pat. No. 6,906,825 to Nakahara et al. describes a halftone processing section that halftone-processes input image data using a plurality of dither threshold planes. An image output section having different output position accuracies between a main scan direction and a sub-scan direction outputs an image corresponding to halftone-processed image data. Each of the dither threshold planes consists of a plurality of the same unit threshold matrixes. In the unit threshold matrix, a relatively medium to high threshold array in a predetermined threshold range corresponding to the entire tone range of the input image data is an aperiodic array and an anisotropic array with neighboring thresholds having close values, in a direction coincident with a scan direction, in which the output position accuracy of the image output means is low. With this configuration, the image output section outputs an image having serial medium and high tone dots in the scan direction.
U.S. Pat. No. 6,178,008 to Bockman et al. describes an automatic system that forms color LUTs (or LUT-forming data) for automated reference—typically in error diffusion (ED). A first aspect is for printers with six or more colorants. Three ramps, for different basic colorants, are photometrically measured; mainly just those results yield a transform from 3D color to system colorants. In a second aspect, some device-state candidate colors are chosen for black replacement. In a variant the choice is subject to (1) maintaining some chromatic colorant in each pixel with black; or (2) modifying use patterns to avoid alternative use of composite black vs. black; or (3) adjustments to allow for composite nonequivalence to black. A related third aspect allows replacement only if there is a given minimum amount of composite. In a fourth aspect, candidate states are dropped that have small changes in number of quanta per pixel, or no companion light colorant quantum with each dark one, best eliminating those with too many quanta of each or all colorants. In a related fifth aspect, one state is assigned to each major entry based on, at a gamut surface except at the dark end, favoring states nearer the surface over those nearer a desired major entry; and at the neutral axis, especially its dark end, favoring real black. Other assigning is best done by entry nearness. In a sixth aspect preferably 1D LUTs are formed for finding major entries based on an input-color spec, not monotonic in entry assignment to indices; precomputed ED distributions attach to indices. In a seventh aspect a state LUT formed to access states based on input specs is used to print nominal neutral colors and measured results used to adjust access. In an eighth facet related to the first, the ramps correspond to fundamental combinations of single colorants, e. g. secondaries.